Digital DJ Pool accepts WAV files too. If you upload an MP3, use constant bitrate (CBR) at 320 kbps.
Render an MP3
Choose File > Render. Set Render Bounds to the entire project or to the time selection you want to export.
Under Output format, choose MP3 (encoder by LAME project).
Set Mode to Constant bitrate (CBR), then set Bitrate to 320 kbps.
Choose the output directory and filename, then render the file.
If MP3 is unavailable in your installation, check the REAPER User Guide for encoder setup for your operating system and REAPER version. Encoder availability can vary by platform and build.
